For decades, performance enhancement of microelectronic chips has been obtained by scaling down feature size of silicon (Si) single devices resulting in a concomitant improvement of the drive current and an increase of device density. However, because this strategy finds its limits around the 22 nm node, new channel materials like e.g. germanium (Ge) with intrinsically higher mobility than Si have to be introduced to further improve efficiency of circuits.
Because of the low availability of Ge on earth, it is most likely that mass production will occur on hybrid substrates where only a thin Ge layer is present on a Si substrate serving as mechanical support. For example, Ge can be deposited epitaxially on Si by chemical vapor deposition (CVD). However, because of the large 4% lattice mismatch between the two materials, coherent growth is only limited to less than 1 nm. Convenient layers are always thicker and thus present strain relaxation which mainly occurs via the formation of misfit dislocations at the interface between Si and Ge. These misfit dislocations can terminate at the edge of the wafer but often penetrate through the Ge film and end at the surface as a threading dislocation. It is crucial to avoid defects because of their adverse effects on the electrical performance of devices but also on their reliability.
Therefore, characterization techniques are needed to monitor their density and distribution so as to develop growth techniques minimizing the occurrence of such defects. Unlike, for example, Transmission Electron Microscopy (TEM) or Electron Beam Induced Current (EBIC), defect etching is a simple, fast and low cost technique to assess crystal quality of materials (see M. W Jenkins, J. Electrochem. Soc., Volume 124, Issue 5 (1977), 757-762).
Wet etching of semiconductor materials is usually conducted in a three component chemical mixture comprising an oxidizing agent that oxidizes semiconductor surface atoms (e.g. HNO3, H2O2, O3, Br2, CrO3, K2Cr2O7), a complexing agent that dissolves the oxide that is formed on the surface (e.g. HF) and a solvent for dilution (e.g. H2O, CH3COOH) (see M. W Jenkins, J. Electrochem. Soc., Volume 124, Issue 5 (1977), 757-762). In the case of defect etching the overall reaction is sensible to the presence of defects at the surface, i.e. it is sensible to the difference in stress level and/or composition variation so that the etching proceeds faster or slower at the defect site than in the perfect crystal, or in other words at locations where no defects are present. As a consequence, because the surface is originally relatively smooth before etching, some topography (e.g. pit or hillock) is created on the etched surface and the defects are rendered visible. Microscopy techniques such as Scanning Electron Microscopy (SEM) or Atomic Force Microscopy (AFM) then allow observation of the etched surface and the determination of the nature of defects and their density.
For Ge, depending on the surface orientation of the substrates (100, 110 or 111) some solutions are available for the revelation of threading dislocations. However, the etch rate of these solutions is in the order of a few μm·min−1 which renders the revelation of defects in thin layers impossible. By diluting the solutions, the etch rate may be decreased but the selectivity towards defects is also drastically decreased. As a result, the overall process shifts from a surface reaction controlled regime to a mass controlled regime, or in other words, from a defect preferential etching to a polishing etching which makes it impossible to reveal the defects.
In “L. Souriau, V. Terzieva, M. Meuris and M. Caymax, Solid State Phenomena Vol. 134 (2008), 83-86” a solution is described for revealing threading dislocations in thin Ge layers. The solution described in this document is based on a CrO3/HF/H2O system which was also extensively described for Si and GaAs (see J van de Ven, J. L. Weyher, J. E. A. M. van den Meerakker and J. J. Kelly, J. Electrochem. Soc., Volume 133, Issue 4, (1986), 799-806). The solution exhibits a low etch rate of between 7 and 100 nm·min−1 depending on the doping, strain state and surface orientation of the Ge layer. It has a good selectivity towards defects. With this solution it is possible to reveal dislocations on (100) and (111) oriented Ge.
A disadvantage of the CrO3/HF/H2O solution described in these documents is that it makes use of carcinogenic CrVI together with HF acid, which makes it environmentally and user unfriendly.